Auto-focus keeps going haywire.

Quick bit of technical help needed if anyone has any ideas. My autofocus started going wonky yesterday - was shooting action stuff so a pita. It would go way out of focus than try a couple of times to focus back, then give up - so I'd have to switch to manual focus, get it as near as - then switch it back to auto focus where it would start doing the micro adjustments.I tried it at things different distances and it happened each time. It's a Canon 650D with a Sigma 18-300. Where does the distance sensor reside - is it inside the lens? ...or anything else that could be causing it?

Andy, test all your lenses. Is the siggie a new addition to your gear, or has it (or the body or even both) been dropped lately? Are the AF contacts clean on both body and lens? And are you getting any error msg? If yes, it could need checking by a service centre.If the answer is no, and it was a bland grey day, then that could be the answer, lack of contrast to lock focus onto. Best advice is to check every lens you have, and let us know what that throws up.

Thanks Colin, I've tried it with two other lenses - both Canon and it's happening with them too. Was quite well lit/contrasty when I tested them. The contacts look OK and I've given them a wipe with a little dab of cleaner on a cloth and dried properly. Do you know where the sensor is for the autofocus?

The AF sensor(s) in most DSLRs is located in the "floor" of the mirror box. The mirror you see when you have the lens off is actually semi-transparent and there's a second mirror behind it that redirects the image to the "phase detection" sensors that are located down below. I believe your 650D has 9 AF points (sensors) and that all of them are the higher performance "dual axis/cross type". Some cameras only have one or two or three of that type and the rest are lower performance "single axis" sensors (some oriented vertically, some oriented horizontally).

More often than not, problems where AF performance has been okay, but drops off with multiple lenses, it's a problem with the electronic contacts needing cleaning. Did you use isopropyl alcohol (also called "rubbing alcohol") to lightly dampen a rag and wipe the contacts on the rear of the lens? Finger oils and similar on them can interrupt the very low voltages used to communicate between camera and lens.

If it were just one lens fouling up, I'd suspect that it's AF system had some fault. That's DOES happen. But if you are seeing it happening with all your lenses, it's unlikely lens-related, more likely camera-related. It might be as simple as a large chunk of dust has settled over the AF sensors and is partly obstructing them. If you have a "bulb blower" to gently puff away dust from the camera, remove the lens and try that... But DON'T go poking around inside there. It's all too easy to damage the mirror or the focus screen or the shutter. If a gentle puff of air doesn't clear things out, leave it to a professional to check and clean the camera, then test the AF system.

Another all-too common problem is incorrect camera setup.

First, your Canon has three different autofocus modes to select between: One Shot, AI Servo and AI Focus. One shot is for stationary subjects. It acquires focus, stops and locks, and gives you Focus Confirmation. AI Servo is for moving subjects. It continuously updates focus and never stops (until you release the button). Since it never stops, there's no Focus Confirmation with AI Servo. Finally, AI Focus isn't really a focus mode at all. It's automation. The camera is supposed to decide for you whether or not the subject is moving, then switch to use either One Shot or AI Servo, whichever is appropriate. This seems to cause a slight delay and doesn't always choose correctly or may not switch properly if a subject starts or stops moving. I avoid AI Focus entirely. It also might be a clue that the most pro-oriented Canon cameras don't even have AI Focus choice... they only have One Shot and AI Servo. Note: If you select a scene mode such as "Sports" ("running man" icon), those are "super automated" and will likely override any AF settings and may force you to use AI Focus.

Next, you can choose between All Points/Auto Selection and leave it up to the camera or manually select a Single AF point. The latter is more work for you, to keep it right on the subject, where you want the lens to focus, but is by far the most consistent and accurate method of autofocus. I use Single Point the majority of the time, only make limited use of several different multi-point patterns my cameras offer, in very specific situations where they can work well. (Your camera only has two choices: All Points or Single Point.)

A "do it all" lens such as that 18-300mm typically doesn't have a very high performance AF drive system in the lens itself, either. I looked it up and it appears to be one of Sigma's "micro motor" lenses, which are slower and might hunt more than their higher end, "HSM" or "hypersonic" focus motor lenses (similar to Canon's USM or "ultrasonic").

Ambient light conditions, oblique light striking the front element of the lens (use a lens hood), low contrast or lack of much detail on the subject itself, all can make AF struggle to "lock on" and track subjects, too. That's particularly true with a variable aperture zoom that drops to a relatively small, dim f/6.3 at the longer end of it's range. It can be further exacerbated by adding a filter to a lens, especially one that's fairly dark and further reduces how much light is available for the AF sensors to work with (such as a Circular Polarizer, which reduces light by up to about 2 stops). You didn't mention what other lenses you have, so it's hard to say what may or may not be expected from them. Some of this is simply out of your control, of course. But other things you can work with and change, hopefully improving AF performance.

amfoto1 wrote in post #18492095More often than not, problems where AF performance has been okay, but drops off with multiple lenses, it's a problem with the electronic contacts needing cleaning. Did you use isopropyl alcohol (also called "rubbing alcohol") to lightly dampen a rag and wipe the contacts on the rear of the lens? Finger oils and similar on them can interrupt the very low voltages used to communicate between camera and lens.

PhotosGuy wrote in post #18492097Pure isopropyl alcohol is good. "Rubbing alcohol" contains oils & other things & is not recommended.

Pure isopropyl alcohol may be 'good'. It is also quite impossible to obtain/store/use outside of a vacuum. Even reagent grade isopropanol is only 99.5% pure. The stuff is EXTREMELY hygroscopic. Even if you manage to obtain some, the moment you open it to the environment, it ceases to be pure. Add to that the fact that it is twice as dense as air, extremely flammable, and contact toxic, and I'd start looking for something a bit safer.

Bassat wrote in post #18492201Pure isopropyl alcohol may be 'good'. It is also quite impossible to obtain/store/use outside of a vacuum. Even reagent grade isopropanol is only 99.5% pure. The stuff is EXTREMELY hygroscopic. Even if you manage to obtain some, the moment you open it to the environment, it ceases to be pure. Add to that the fact that it is twice as dense as air, extremely flammable, and contact toxic, and I'd start looking for something a bit safer.

I've never had a problem using drug-store isopropanol (rubbing) alcohol for such things. Acetone also works well. Both will absorb water from the environment, but acetone (fingernail polish remover) stinks, and can remove lacquer sealants on circuitry. Last time I looked for such stuff, Radio Shack sold an isopropanol-based spray cleaner made specifically for electronics (no grease, particulates, other contaminants).

I am sure pure isopropanol would do the job. Less expensive, less dangerous alternatives exist. Your dishes would be very clean if you washed them in alcoholic potassium hydroxide. Most folks find grocery store detergents do an acceptable job. Same principle.

PhotosGuy wrote in post #18492097Pure isopropyl alcohol is good. "Rubbing alcohol" contains oils & other things & is not recommended.

Pure isopropyl alcohol is difficult to get and expensive.

Rubbing alcohol is cheap and widely available. It's 70% alcohol and in the U.S. the rest is usually de-mineralized water. Check the label.

In the UK there are other additives, sometimes including castor oil, wintergreen. Some "boutique" rubbing alcohols in the U.S. have these types of additives, too.

But the "cheap stuff" usually isn't diluted with anything other than water and works fine to clean electronic contacts such as those on lenses. I've been using it for this purpose for a couple decades with no problems at all.

Radio Shack Electronic Contact Cleaner and Lubricant was mentioned above, too... I recommend you not use it for this purposet (or similar, since Radio Shack is out of business now). The key word is "lubricant". I've got a can of it right here and the lubrication is provided by some sort of light oil that doesn't evaporate and remains on the contacts. You don't want that on the lens to camera contacts, where communication is done with very low power electronic signals (works great on switches and such, though).

The best thing to use on some electronic contacts is "De-Oxit". It removes oxidization and really cleans things up well, leaving no residue at all. However, it's expensive. Plus, the electronic lens contacts in Canon cameras are gold plated... and gold is used specifically because it doesn't oxidize. So it would be a waste of money to use De-Oxit on them.

If so turn them off for moving subjects. Even the Canon 1DX and Canon 300 F2.8 L IS can fail very badly with IS on and moving subjects.

That's almost funny... I've been using around a dozen different Canon IS lenses on various Canon cameras for fifteen years and almost never turn it off. With maybe a half million shots taken with IS lenses in that time, I bet I can count the number of times it might have "messed up" an image on my ten fingers. Far, far more often it improves images and, IMO, helps autofocus work better.

I particularly use IS when panning to follow moving subjects... handheld, on a monopod or even on a tripod with a gimbal mount. If I'm using a slower shutter speed to try to deliberately blur down background in a panned shot, I'll switch IS to "Mode 2"... But I still leave it on (some of the newest lenses auto-detect panning and switch to Mode 2 automatically).

We often read or hear that you should "turn off IS on a tripod", but that's over-simplification. There are only five Canon lenses where it's necessary to turn off IS when a lens is solidly locked down on a tripod and there is absolutely no movement. When there is no movement at all, the original 75-300mm IS USM, EF 300mm f/4L IS USM, 28-135mm IS USM, original 24-105mm f/4L IS USM and original 100-400mm L IS USM (push/pull version) can potentially go into sort of a feedback loop where their IS system actually causes image movement and causes image blur. But the other 25 or so Canon IS lenses self-detect and turn off IS on their own. And it's really only when there's absolutely no movement at all, whether that's on a tripod or fully locked down in some other way. Canon themselves advises in their user manuals to turn off IS when using high shutter speeds or on a tripod. But according to Canon USA tech guru Chuck Westfall explains that this is in order to save battery power. But I can tell you from personal experience that IS uses very little power. (Besides, if it self-detects non-movement and turns itself off, it's saving power in the same way).

I have seen the IS system fail in one lens (EF 300mm f/4L). When it did, focus was largely unaffected.

I've also had another lens' focus fail (EF 300mm f/2.8L), but that was another thing entirely.... and IS still worked fine.

The only specific situation I've heard about where IS may get fooled is when you move the camera erratically, like when trying to follow a hockey player or similar. You may then move the lens back and forth in a way that's unpredictable to the IS system. The result can be that IS introduces blur rather than removes it, as it has "bottomed out" in one direction when it needs to move further in that same direction.

Some lenses, like the EF 300 mm f/2.8L IS II USM, have a third IS mode, where IS is active while panning, but doesn't move until you actually press the button to take the picture. Thus you are sure it starts from its center position, and doesn't fail because of being out of moving range.

Bassat wrote in post #18492313I've never had a problem using drug-store isopropanol (rubbing) alcohol for such things. Acetone also works well. Both will absorb water from the environment, but acetone (fingernail polish remover) stinks, and can remove lacquer sealants on circuitry. ....

Be much more careful using acetone. Not only is acetone a great solvent and can remove paint, it is also very hard on some plastics and will leave them a sticky melting mess. There is a reason it makes a great nail polish remover.

To the OP, I doubt that your issue is caused by dirty contacts. More likely the camera is in AI-Servo mode. If you check and make sure it's in one shot and still hunting, one other possible long shot is that some dust/hair has fallen on the AF sensor or one of the mirrors. A couple of puffs with a rocket type blower can make sure that there are no obstructions on the AF sensor. If that doesn't work, I'd start suspecting some type of failire with the AF sensor.

Not sure why, but call me JJ.I used to hate math but then I realised decimals have a point.. ::Flickr::::Gear::

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